Generally speaking, ethylene-vinyl alcohol copolymer ("EVOH") is a popular thermoplastic, due to its gas barrier properties and excellent resistance to organic liquids. However, EVOH does not readily thermoform at the relatively lower temperatures required for thermoforming of many common structural materials, due to undesirable cracking, uneven stretching and the like, particularly at large depths of draw.
Blends of ethylene vinyl alcohol polymers with polyamides in general are known, and have been used in packaging applications as barriers to inhibit the passage of atmospheric oxygen or other gases. Japanese patent application No. 53-49050 discloses a blend of EVOH with 5-40 weight percent polyamide. The polyamides include a copolymer of hexamethylene diamine with isophthalic and terephthalic acids, in mole ratios of 100/0 to 50/50. The blend is formed into a film, which is said to possess excellent gas barrier properties. U.S. Pat. No. 4,990,562 is directed to blends of a major portion of EVOH and a minor portion of amorphous polyamide and preferably also a semicrystalline nylon. However, the blends discussed in the above references do not thermoform very readily at large depths of draw (showing cracks or uneven stretching in parts of the container), using conventional thermoforming processes. The present invention is directed to an innovative process for thermoforming EVOH containers at relatively large depths of draw without many of the problems otherwise commonly associated with such processes.